[MLton] selects re 0 referenced?!

[Stephen Weeks]

> If you notice I select out 2,3,4 instead of 1,2,3. Apparently somewhere
> under the hood, some part of the compiler adds 1 to the select index.
> Since internally, i am assuming, it is 0 indexed. Wow that caused a
> debugging head ache =). I was treating selects as 1 indexed!

Yeah, the one-index is for display only (see Exp.layout in
ssa-tree.fun).  Everything internally is zero indexed, from the front
end on.